Our eyes have difficulty focusing on the blue light, I don't remember the exact reason for this but I think it's something to do with the way the different wavelengths behave when passing through the lens.
Human eye has this fairly simple single lens optical path, and the problem with single-lens optics is they focus different wavelengths to different planes, hence causing "chromatic aberration". Camera optics get around this by using combination of lenses. Human vision system implements a very complex
AI algorithm instead, but there are limits what that can do. Light blue targets contain enough green so human brain can use high-resolution data from green receptors, combined with the color obtained from the blue receptors.
You can actually try this out: sometimes you can see company logo signs that contain deep blue color (it needs to be very deep blue!) which, as a result, looks blurred at night, while adjacent other colors look sharp. You can actually force your eye to focus on that blue (you know, by forcing your eye muscles to defocus the image); then the blue suddenly looks focused, but red and green becomes clearly defocused. This test confirms the weakest link in human vision regarding blue is in the lens, not the receptors (although it is also true the number of blue receptors is lowest; obviously it would be a waste having more!)
CRT projection TVs used to slightly defocus the blue gun, I believe for a similar reason.
No, the reason here isn't similar, but instead about chemistry of the phosphors. In order to compensate for the effect in eye, they would need to project blue to actually
different distance, which would be nearly impossible. Defocusing is done
on tube (adjusting the electrostatic/electromagnetic focusing), not even on the lens. The reason is that the blue phosphors used in the tube are quite weak and can't produce high illumination level; similarly the blue tube is the first one to wear out. But by defocusing the gun, tiny bright spots can utilize a larger area of phosphor, reducing peak intensity, utilizing more phosphor to emit the light. The obvious problem is, the resolution suffers. But lo and behold, didn't we just describe that the human eye sucks on blue resolution (because our eyes focus based on green and red)? So we can
accept that defocused blue tube.